1. Field of the Invention
This invention relates generally to a process for vaporizing a crude petroleum feedstock prior to the thermal or steam cracking of such feedstock to olefins and other petrochemicals. More particularly, it relates to the preheating of such a feedstock, preferably one boiling in the range of a vacuum gas oil or higher, in one or more stages, in the convection section of a conventional, tubular (steam) cracking or pyrolysis furnace.
2. Description of the Prior Art
Production of olefins in general and of ethylene in particular has been achieved in the past by thermal cracking of a crude petroleum hydrocarbon feedstock and rapidly quenching the cracked effluent, e.g. in a transfer line (heat) exchanger. During the last two decades or so, the trend has been to use heavier and heavier feedstocks than the ethane or naphtha feedstocks once used predominantly. However, the use of such heavy feedstocks, e.g. vacuum gas oils and those boiling higher, i.e. a heavy residuum fraction having an initial boiling point above 230.degree. C., has led a variety of operating problems, the foremost one of which has been coke formation. It has been necessary to preheat the heavy oil or any liquid hydrocarbon feedstock to a reaction inlet temperature of about 600.degree. C. Conventionally, the preheating of the heavy hydrocarbon feedstock is achieved by heating it in the convection section of the ordinary tubular pyrolysis or thermal cracking furnace to a temperature of about 200.degree. C. to about 260.degree. C., or, alternatively, by heating such a feedstock in indirect heat exchange relationship to about 225.degree. C. to about 260.degree. C. The heated liquid is then mixed with superheated steam and externally flashed, i.e. outside the convection section, to 600.degree. C. from the vaporization mix temperature of 380.degree. C., or it is separated from the vapor phase and vaporized externally in a flash drum by being contacted with superheated steam or a preheated mixture of steam and vapor phase feed. These methods of external flash vaporization have been done to avoid convection section coking, and have been well documented in U.S. Pat. No. 3,617,493; 3,718,709; and 4,264,432.
U.S. Pat. 4,264,432 specifically recites the features of external mixing of the preheated hydrocarbon feedstock with superheated steam followed by flashing.
U.S. Pat. No. 3,617,493 discloses the use of an external vaporization drum for the crude oil feedstock and recites the use of a first flash wherein the overhead vapor is naphtha and of a second flash in which the overhead vapor is a gas oil boiling between 230.degree. C. and 600.degree. C. Residual liquids are removed, stripped with steam, and used as fuel.
U.S. Pat. No. 3,718,709 discloses a pyrolysis process that is designed to minimize the coke deposition in the radiant coils. It specifically discusses the preheating of heavy oils to an extent of vaporization of about 50% with superheated steam and the separation of the residual liquid at temperatures approximating 300.degree. C.-450.degree. C. In column 3, lines 6-9 of this patent, it is expressly stated that:
"The composition of the feed (steam: hydrocarbon) is to be maintained within the limits (of 0.5-5.0) in order to avoid deposits of coke in the furnace tubes."
The solutions to the problem of coking formation and deposition through the measure of external flash vaporization, such as that proposed by the above three U.S. patents, are, however, quite costly in that they require increased costs of equipment and piping, owing to the fact that they have to be constructed of expensive alloys. Moreover, owing to the difficulties in controlling the flows of the hot vapor and liquid streams, an individual mixer flash drum system might have to be provided for each radiant heating coil used in the pyrolysis furnace. For a furnace with multiple radiant coils, this would substantially increase the investment cost of each furnace.
The present invention, however, offers an economically advantageous alternative to the external flash vaporization systems and methods to avoid convection section coking. It does not require increased equipment and piping costs, nor does it suffer from the dead space inherent in a flash drum design which promotes more than the usual amount of coke formation which, once formed, is a tarry material that is very difficult to remove from the drum and to discard.
The advantages of this invention are achieved through the use of a small, critical amount of hydrogen in the convection section to inhibit the polymerization reaction of the hydrocarbons preheated therein, thereby inhibiting coke formation in the convection section tubes resulting from such polymerization reaction. Such coke formation not only limits heat transfer in the convection section, it also increases the pressure drop throughout the whole system. The increased pressure drop causes premature shut-down of the furnace and, concomitant therewith, decreased production, thereby decreasing the profitability of the furnace operation.
Use of a small, critical amount of hydrogen in the convection section during the preheating of the crude (heavy) petroleum feedstock is not to be confused with hydrogenation, hydrocracking, or other downstream reactions in which extensive amounts of hydrogen are present, with or without a catalyst also present, to promote pyrolytic cracking of the feedstock to lower molecular weight hydrocarbons and/or to eliminate sulfur, nitrogen, asphaltenes, and metals such as Ni, V, Na, Fe, and Cu that may be present in the charge, and/or to hydrogenate the aromatic constituents present in the charge.
Thus, for example, U.S. Pat. No. 3,842,138; 3,898,299; 3,907,920; 3,919,074; and 4,285,804 all disclose the use of large excesses of hydrogen for the above purposes.
U.S. Pat. No. 3,842,138 discloses a method of thermal cracking of hydrocarbons under pressure and in the presence of an excess of hydrogen. The excess hydrogen is defined as a molar concentration of hydrogen in the effluents of at least 20% at a pressure between 5-70 bars, a temperature above 625.degree. C., and a residence time of less than 0.5 second.
U.S. Pat. No. 3,898,299 describes a two-stage process for the production of olefins wherein residual oil feedstocks are catalytically hydrogenated prior to thermal cracking of a distillate fraction of the liquid phase separated from the hydrogenated product. Excess hydrogen, described as about 5 to 10 times the molar rate of the residual feedstock fed to the hydrogenation zone, is disclosed.
U.S. Pat. No. 3,907,920 discloses another two-stage process for producing ethylene comprising an integrated hydro-pyrolysis-cracking process wherein the preferable hydrogen/hydrocarbon oil mole ratio for the so-called hydropyrolysis is in the range of about 3/1 to 30/1.
U.S Pat. No. 3,919,074 discusses the conversion of hydrocarbonaceous black oils into distillate hydrocarbons wherein hydrogen is admixed with the black oil charge stock by compressive means in an amount generally less than about 20,000 SCFB, preferably in an amount of from about 1,000 to about 10,000 SCFB.
U.S. Pat. No. 4,285,804 discloses a catalytic hydrotreatment of hydrocarbon oils boiling above 350.degree. C. which is conducted under a partial hydrogen pressure usually in the range of from 50-200 bars, preferably from 90-150 bars; a temperature between 350.degree. C.-470.degree. C., preferably between 380.degree. C.-430.degree. C.; and a residence time for the liquid charge within the reactor of between 0.1-4 hours, preferably between 0.5-2 hours.
All of these last-enumerated U.S. Pat. No. 3,842,138; 3,898,299; 3,907,920; 3,919,074; and 4,285,804 therefore have to deal with excessive amounts of circulating hydrogen that have a heavy impact on the utilities consumption and investment costs of the olefin plant in which they are used. For example, high hydrogen amounts involve the circulation of large volumes of a hydrogen-containing stream for which compression thereof between 20-40 bars is necessary for its fractionation, thus involving prohibitive costs. In contrast, the small amount of hydrogen required in the case of the present invention only has a very small impact on utilities consumption and investment costs because the hydrogen is not needed to reduce the vaporization temperature of the charge but only to inhibit the polymerization of the small amount of olefins created in the convection section and thus reduce the coke precursor. Furthermore, little or no modification of the convection section is required in order to make use of the present invention, and such invention also makes it possible to eliminate the flash drum. Furthermore, use of the present invention can decrease the fouling rate in the transfer line exchanger employed to quench the cracked effluent of the furnace, owing to the presence of a higher concentration of hydrogen in the furnace effluent. However, the degree of improvement is dependent upon the amount of hydrogen added.